There has been a wide-spread use of ultrasonic wave for measurements of metallic microstructures or material properties. For instance, in the PTL 1 below, there is disclosed a grain size measuring apparatus for measuring grain sizes in a steel plate, using the principle that energized ultrasonic waves, transmitted through a steel plate, have different attenuation characteristics depending on grain sizes in the steel plate.
It is known that grain sizes, attenuated ultrasonic waves, and ultrasonic frequencies generally follow a scatter law to be defined (by expression 1), such that:
                    [                  Math          ⁢                                          ⁢          1                ]                                                            D        =                              (                                          K                                  -                  1                                            ·              α              ·                              f                                  -                  n                                                      )                                1                          n              -              1                                                          (                  expression          ⁢                                          ⁢          1                )            
Here, denoted by a is an attenuation rate (dB/mm) of an ultrasonic wave, D is a grain size (mm), f is a frequency (MHz) of the ultrasonic wave, and n is a coefficient representing a scatter mode, typically within 1 to 4 or near.
In other words for ultrasonic waves scattered at crystal grains, the attenuation due to the scattering is promoted as the frequency becomes high. This tendency has an increased significance as the grain size increases. Accordingly, resultant differences in the attenuation rate can be based on to measure qualities of a metallic material, including the grain size, for instance.
For use to energize ultrasonic waves in a material to be measured, there are available known methods including a method employing a piezoelectric vibrator (as a first method), a method employing electromagnetic forces (as a second method), and a method employing a pulse laser beam (as a third method). Among them, the first method needs to closely attach the piezoelectric vibrator to the material to be measured, with an intervening medium (liquid) having a matched acoustic characteristic. Moreover, energized ultrasonic waves need to have frequencies typically about a few MHz or less. The second method permits ultrasonic waves to be energized in a non-contact manner, with a limited spacing (a stand-off distance) typically about a few mm from the material to be measured. Besides, this measuring material has to be a magnetic body. Namely, the second method is inapplicable to inspections such as that of a carbon steel (having a hot austenite structure) in a hot processing, that is a non-magnetic body, or of a stainless steel that also is a non-magnetic body.
In comparison with them, the third method is wide-spread because of advantages permitting non-contact measurements, large standoff distances (several 100 mm), and measurements of non-magnetic bodies.